The present invention relates to acrylic-functional silicone compositions. More particularly, the present invention relates to acrylic-functional silicone-resins which are curable upon exposure to ultraviolet radiation or upon heating in the presence of a free radical type catalyst to compositions useful as conformal coatings or as coatings for optical fibers.
Organopolysiloxanes containing acrylate groups which cure upon exposure to ultraviolet radiation or upon heating in the presence of a free radical type catalyst are known in the art. Generally, these compositions are useful only as release coatings because of their cheesy or crumbly nature. To the best of applicant's knowledge, curable acrylate-functional organopolysiloxanes which are useful for making conformal coatings or as coatings for optical fibers (i.e. they possess the requisite physical properties) are unavailable in the art. Accordingly, there is provided by the present invention solventless, acrylate-functional silicone resins which are curable upon exposure to ultraviolet radiation or upon heating in the presence of a free radical type catalyst to a composition useful as a conformal coating or as a coating for optical fibers.
Merker, U.S. Pat. No. 2,793,223, discloses siloxanes of the formula: ##STR2## wherein R is a hydrogen atom or methyl, R' is a monovalent hydrocarbon radical, and n has a value from 1 to 2, inclusive. Such acryloxy substituted siloxanes can optionally be copolymerized with siloxanes of the formula: ##STR3## wherein R" is a monovalent hydrocarbon radical, an acetoxymethyl radical, or a halogenated monovalent hydrocarbon radical, and m has an average value from 1 to 3, inclusive. Consideration of the teachings of Merker reveals that trifunctional units having acrylate-functional groups bonded thereto as well as tetrafunctional siloxy units are not contemplated.
Merker, U.S. Pat. No. 2,922,806, expands upon the teaching of U.S. Pat. No. 2,793,223, in that there are provided organopolysiloxanes containing polymeric units of the formula: ##STR4## wherein n is an integer from 2 to 4, inclusive, y is an integer from 0 to 1, inclusive, R is a hydrogen atom or a methyl radical, and R' is a monovalent hydrocarbon radical free of aliphatic unsaturation. Again, such acryloxy substituted siloxanes can optionally be copolymerized with siloxanes of the formula: ##STR5## wherein R" is a monovalent hydrocarbon radical or a halogenated monovalent hydrocarbon radical, and m is an integer from 0 to 3, inclusive, the average value of m being from 0.8 to 3, inclusive. Analysis of the specification and examples reveals that the presence of tetrafunctional siloxy units is not contemplated and that at least about 33 mole % difunctional siloxy units must be present in the composition.
Ohto et al., U.S. Pat. No. 3,782,940, discloses organopolysiloxanes containing at least one photopolymerizable organic silicon radical represented by the general formula: ##STR6## wherein R.sup.1 is a hydrogen atom or an unsubstituted or halogen-substituted phenyl radical; R.sup.2 is a hydrogen atom or a methyl radical; R.sup.3 is an unsubstituted or halogen-substituted divalent hydrocarbon radical having from 1 to 10 carbon atoms; R.sup.4 is an unsubstituted or a halogen-substituted monovalent hydrocarbon radical having from 1 to 10 carbon atoms, X is a hydroxyl radical or an alkoxy radical having from 1 to 4 carbon atoms, and a and b are each equal to 0, 1 or 2, with the proviso that a+b equals 0, 1 or 2. Reference to the specification and examples makes clear that tetrafunctional siloxy units are not contemplated and that the difunctional siloxy unit concentration must be at least about 40 mole percent. U.S. Pat. No. 3,865,588 to Ohto et al. discloses substantially the same chemistry as the aforesaid U.S. Pat. No. 3,782,940.
Martin, U.S. Pat. No. 3,878,263, discloses organopolysiloxanes of the general formula: ##STR7## wherein R is selected from hydrogen and monovalent hydrocarbon radicals having from 1 to 12 carbon atoms, R' is selected from monovalent hydrocarbon radicals, halogenated monovalent hydrocarbon radicals, and cyanoalkyl radicals having from 1 to 18 carbon atoms, R" is selected from the group consisting of a divalent hydrocarbon radical having from 2 to 18 carbon atoms, and the corresponding divalent hydrocarbon radical containing C--O--C linkages, R'" is a radical selected from the group consisting of R""O.sub.0.5 and R'SiO.sub.0.5, Z is a group selected from OR"". R"" or OSiR'.sub.3, in which R"" is selected from hydrogen and monovalent hydrocarbon radicals, a and b are each numbers from 1 to 20,000, c is a number from 0 to 3, and e is a number from 0 to 2, with the proviso that when c is zero, then at least one Z must be OR"". Martin does not contemplate including tetrafunctional siloxy units in the composition nor, in view of the examples, does he contemplate less than about 60 mole percent difunctional siloxy units.
Ohto et al., U.S. Pat. No. 3,886,865, relates to photopolymerizable organopolysiloxanes of the general formula: ##STR8## wherein R.sup.1 is hydrogen, an unsubstituted or halogen-substituted phenyl radical, R.sup.2 is a hydrogen atom or a methyl radical, R.sup.3 is a divalent hydrocarbon radical having from 1 to 3 carbon atoms, R.sup.4 is methyl or trifluoropropyl, a is 0 or 1, and m and n are positive integers with the proviso that n exceeds 25, while n/l and n/m are 25 to 20000 and 2.5 to 50, respectively. From the ratios of n/l and n/m it is clear that the trifunctional siloxy units are not present in a large amount and, further, tetrafunctional siloxy units are not contemplated at all.
Cully et al., U.S. Pat. No. 4,201,808, describes photocurable organopolysiloxanes useful as release coatings comprising an organopolysiloxane containing an average of at least one acryloxy and/or methacryloxy group per molecule, a low molecular weight polyacryl crosslinking agent, and, optionally, a photosensitizer. The organopolysiloxanes having acryloxy and/or methacryloxy groups have the average formula: ##STR9## wherein m has an average value greater than about 25, R is, independently, acryloxy, methacryloxy, an unsubstituted monovalent hydrocarbon radical having from 1 to 20 carbon atoms, or a substituted monovalent hydrocarbon radical wherein the substituents are selected from the group consisting of chloro, fluoro, cyano, amido, nitro, ureido, isocyanato, carbalkoxy, hydroxy, acryloxy and methacryloxy, and n has an average value from about 1.8 to 2.2; said organopolysiloxane containing an average of at least one R group which contains an acryloxy or methacryloxy group. Reference to the specification and examples makes clear that the acryloxy-functional polysiloxanes are comprised of substantially all diorganosiloxy units (e.g. free of trifunctional and tetrafunctional siloxy units).
Eckberg, U.S. Pat. No. 4,348,454, discloses acrylic functional polysiloxanes which are especially useful for ultraviolet curable silicone release coating applications. As in all release coating applications the polysiloxane consists essentially of diorganosiloxy units and thus is not useful as a conformal coating.